I’m trying to make the scroll position of my Composable available in the View world, but I’m having ...

# compose

[zsperske]

I’m trying to make the scroll position of my Composable available in the View world, but I’m having trouble.

//I create a scroll state in my view model and pass it here to be used
val scrollState = rememberSaveable(saver = ScrollState.Saver) { viewModel.state.scrollState }

//Add to my column
Modifier.verticalScroll(scrollState)

//In on save instance state of an Android View I grab the state and save it to a bundle
savedState.scrollPosition = viewModel.state.scrollState.value

However, the scroll state’s value is always 0. Am I going about this all wrong?

[chatterInDaSkull]

My hunch is you are missing a rememberScrollState() somewhere

[zsperske]

I had tried to do that manually by doing:

val scrollState = rememberSaveable(saver = ScrollState.Saver) { viewModel.state.scrollState }

[zsperske]

I can’t do rememberScrollState since I need to create the scroll state in my view model

[chatterInDaSkull]

can you undo your rememberSaveable and simply use rememberScrollState() and manually set viewModel.state.scrollState = scrollState.value and try it out?

probably not good for performance and might incur a lot of recompositions but still might help validate something

[zsperske]

interesting that does capture the value correctly

[zsperske]

but I agree that this isn’t optimal, I don’t want it to solve it this way

[chatterInDaSkull]

I don’t think this is entirely wrong fyi, you could set your view model state by using derivedStateOf to optimize a bit

Out of curiosity, what does your view model look like with respect to the scroll state? how is it defined?

[chatterInDaSkull]

also i’m not sure if you know but rememberSaveable(saver = ScrollState.Saver) { viewModel.state.scrollState } is only giving it an initial state, I think it creates new scroll state objects (once again easily verifiable)

[zsperske]

view model looks like this

class ViewModel() {
   var state by mutableStateOf(MyState())
    private set

   fun updateScrollPosition(position: Int) {
      state = state.copy(scrollState = ScrollState(position)
   }
}

data class MyState(val scrollState: ScrollState, ...)

To test your theory I temporarily made the scrollState a var which breaks recomposition. But if I were to change it to instead have a updateScrollState method I think I’d end up in an endless loop of recomposition

[zsperske]

(testing that now)

[zsperske]

while things get saved correctly, and I make a call to update the scroll position after restoring the state, the view doesn’t scroll

[zsperske]

val scrollState = rememberScrollState(viewModel.state.scrollPosition)
viewModel.updateScrollState(scrollState)

[chatterInDaSkull]

are there many many recompositions per scroll event?